Various rubber-based compositions commercially available on the market today require the presence of at least one tackifying resin to increase adhesion. Such resins will be referred to hereinafter for simplicity as "tackifiers".
Since the invention is particularly directed to rubber-based adhesives, e.g. pressure-sensitive adhesive coatings and the like, the nature and objects of the invention will be described for purposes of illustration by reference thereto.
In general, rubber-based adhesives may be classified broadly as either natural rubber or synthetic rubber, e.g. butyl rubber. Natural rubber adhesives are the more common, employed in various products such as finger bandages, or surgical adhesive tapes, industrial tapes and the like. Synthetic rubbers such as butyl rubbers are commonly employed in industry for such uses as pipewrap tapes for protecting pipelines against corrosion, etc.
Common to both types of rubber is the initial step of mixing chunks of the rubber with other ingredients of the mix (fillers, antioxidants, plasticizers and the like) in an internal mixer, e.g. a Banbury, to provide a molten "premix" which is then transported from the Banbury for further processing.
With respect to natural rubber adhesives, it would he most desirable from a manufacturing standpoint also to incorporate all of the needed tackifier in the Banbury or other internal mixer, thereby admixing all of the ingredients in single processng step. However, it is not possible to do so. While it is entirely possible to incorporate a portion of the required tackifier in the Banbury (as seen from the illustrative example that follows), attempts to add all of the required amount of tackifier in this manner result in a mixture so adhesive in nature that it cannot be dropped from the mixer. The only non-solvent procedure known commercially to incorporate the tackifier is on a mill. Consequently, commercial processes for preparing natural rubber adhesives in a solid state prepare an initial natural rubber premix in a Banbury or other internal mixer and then transport the premix to a milling operation where the resin is scooped or shoveled onto a two-mill roll where it is mixed into the premix, for, say, 10-30 minutes to prepare the adhesive formulation.
Following the milling operation, the adhesive formulation may then be transported to the final steps of the manufacturing process. For example, it may be transported to a calendering operation for coating onto a backing material to prepare an adhesive tape.
Apart from the disadvantages inherent in such a batch process, the necessity to employ a mill has its own inherent disadvantages. A conventional two-roll mill for this purpose may be on the order of five feet or more in length and takes up a considerable amount of space in the manufacturing process. Manpower is required to apply the resin and supervise the milling operation. Resin gets on the floor, sticks to one's shoes and creates environmental problems. Finally, there is the safety factor. While serious plant injuries are few, by far the most prevalent cause of those which do occur is the milling operation. Because of human error, a worker may on occasion get an arm caught in the mill, resulting in extremely serious and often tragic injury.
Synthetic rubber adhesives may be made by the same general process. For example, in the earliest production of butyl adhesives, the initial premix of the rubber components, e.g. virgin butyl, halogenated butyl and/or reclaimed butyl rubber was first made in the Banbury and then transported to the two-roll mill for incorporation of the tackifiers. Subsequently, this process was modified to incorporate the tackifiers in the Banbury. While this eliminated the step of adding the tackifier at a second (milling) station, it also had certain inherent disadvantages. The tackifiers had to he incorporated sequentially, rather than incorporating all at once (typically three stages) and the output time was materially lengthened. Consequently, to increase output it was found necessary to add additional Banbury mixers which occupy a large amount of space and at present require a capital expenditures of on the order of $2,000,000.
To summarize, natural and certain synthetic rubber adhesives require a milling step to add the tackifiers. Butyl and certain other synthetic rubber adhesives can be prepared by incorporating the tackifier in the Banbury. However, the output is slow and a better way (other than milling) is extremely desirable in commercial production.
The task of this invention can accordingly be said to be to devise a better system for incorporating tackifiers into adhesives such as the heretofore described rubber-based adhesives.